The invention relates generally to a method of shielding a spacecraft from damaging collisions with ions, and in particular to electrically-biased, high-Z material reflectors used to actively neutralize and redirect incident ions away from the spacecraft.
A major issue in the design of spacecraft is the mitigation of the effects of damaging contamination. The three primary sources of contamination of interest are: deposition of foreign material on the spacecraft surface; erosion of spacecraft surface material through collision with foreign material; and changes to the electrical potential on, or in the vicinity of, the spacecraft via the collection of electrically charged foreign material.
There are a multitude of effects that can arise as a result of spacecraft contamination. For example, contamination to optical surfaces can degrade imaging capability or reduce the power production capability of solar arrays. Contamination to thermal surfaces can change the thermal balance of the spacecraft. Eventually the concomitant changes in the spacecraft operating temperature can damage electronics. Contamination from electrically charged material can result in spacecraft electrical failures or the electrical shorting of the power-generating solar arrays.
There are seven primary sources of contaminant ions (see FIG. 1): ions from the space plasma in the vicinity of the spacecraft; neutrals in the ambient space atmosphere ionized through collision or ultra-violet (UV) radiation from the sun; neutrals out-gassed from the spacecraft becoming ionized; neutrals exhausted from spacecraft thrusters that become ionized through collision or UV radiation; high-energy Ions exhausted from spacecraft thrusters on a direct impingement trajectory with spacecraft surfaces; slow ions near the thruster exit plane that are formed through charge-exchange collisions with ambient neutrals or neutrals from the thruster; and neutrals sputtered from spacecraft surfaces due to ion impingement. These neutrals can then become ionized through collision or UV and accelerate back into the spacecraft surface.
Once these ions are in the vicinity of the spacecraft (within about 4 Debye lengths), they will be accelerated into the spacecraft surface due to an electric potential difference between the spacecraft and the surrounding plasma. This acceleration will give the ion sufficient energy to sputter away spacecraft material causing contamination in the form of erosion. These sputtered neutrals can also form new ions, thereby self-fueling the ion contamination process.
A source of ions that deserves specific attention is ions emitted from the spacecraft propulsion system. As an example, consider the case where the source of the contaminant ions is an on-board Hall-effect plasma thruster. The thruster will act to accelerate xenon ions to about 300 eV. Ideally these ions will travel in a straight trajectory away from the spacecraft without collision. In reality these ions will be accelerated in a cone with about a 45-degree half angle and have slightly curved trajectories in response to collisions with other fast ions or interactions with the earth's magnetic field. These ions will also experience a substantial number of charge-exchange collisions with neutral xenon gas at the thruster exit plane. These collisions result in a fast neutral xenon atom and a slow xenon ion.
The fast ions generally affect the spacecraft through direct impingement on surfaces, which can cause material sputtering and spacecraft charging. The state-of-the-art method to preclude this fast ion impingement is to place the thrusters so that the plume will not directly impinge a critical surface. This is a major hindrance to spacecraft operation. For example, the xenon ion thrusters used for stationkeeping on geo-synchronous communications satellites would optimally be directed north and south. Instead the thrusters are canted away from the north-south direction as much as 45 degrees to avoid direct impingement of the plume on spacecraft surfaces. This cant angle decreases the effective thrust delivered in the required direction by about 30%, which means that the spacecraft must carry additional propellant to offset the thrust loss.
The slow ions would seem to be less of a concern due to their lower kinetic energy. However, they are more prevalent in the critical region near the spacecraft, and their low kinetic energy makes them more likely to be accelerated back into the spacecraft by the local electric field. There is no demonstrated state-of-the-art method to avoid the contamination from these slow ions. The spacecraft designer must instead increase the beginning-of-life specifications for critical capabilities such as power generation and thermal control, so that the spacecraft retains sufficient operating capability at end-of-life. The impact of the added capability at beginning-of-life is added mass in non-revenue-generating components.
One method for actively shielding the spacecraft from contaminant ions uses a series of baffles between the exit plane of the plasma thruster and critical spacecraft components. The baffles use a louvered design to force an incoming ion to collide with the baffle surface. No electrical bias is applied to the baffles. The baffles are designed solely to be a material block between the contaminant source and selected critical areas of the spacecraft.
The NASCAP (NASA Charging Analysis Code) and similar development under AFRL and NASA have considered related spacecraft charging effects. NASCAP is used to predict spacecraft charging due to the space environment. As part of this analysis NASCAP calculates the electric potential of the spacecraft, which is primarily determined by surface area and solar array voltage. In its analysis, NASCAP does consider the effect of this spacecraft charging on the trajectories of local ions and electrons. Further, NASCAP has undoubtedly been used to predict changes in the ion and electrons trajectories (and therefore changes in the spacecraft ion impingement) caused by changing the spacecraft electrical potential. However, the NASCAP work has never considered the unique concepts disclosed herein whereby electrically-biased, high-Z materials can be used to actively neutralize and redirect incident ions away from the spacecraft. NASCAP is an analysis tool that may, however, be helpful in the detailed design of the disclosed concept.